System and method for inductively charging a battery

ABSTRACT

An inductive charging system for recharging a battery. The system includes a charger circuit and a secondary circuit. The secondary circuit includes a feedback mechanism to provide feedback to the charger circuit through the inductive coupling of the primary coil and the secondary coil. The charger circuit includes a frequency control mechanism for controlling the frequency of the power applied to the primary coil at least partly in response to the feedback from the feedback mechanism.

BACKGROUND OF THE INVENTION

Charging of batteries with an inductive power supply is well-known.Inductive charging of batteries for electric automobiles as well as thecharging of small electric appliance batteries such as those fortoothbrushes has met some amount of success. Because inductive chargingdoes not require a physical connection between the battery and thecharger, the charging is considerably more convenient. However, not allbatteries are easily charged inductively. Lithium-ion batteries (Li-Ion)are one such type of battery.

Recharging Li-Ion batteries is not as straightforward as that of otherbatteries. Li-Ion batteries are unable to absorb an overcharge. If aconstant current is applied to a fully charged Li-Ion battery, metalliclithium plating may develop which could lead to failure of the battery.Thus, care should be taken not to overcharge the battery. Conversely,charging a Li-Ion battery to full capacity presents some difficulty. Themaximum voltage of a Li-Ion battery can be attained relatively quicklyduring recharging by applying a constant current to the battery.However, when the Li-Ion battery reaches a maximum voltage, the Li-Ionbattery may not be fully charged. Without further charging, the batterywill only be approximately 65% charged. If a constant current iscontinually applied to the battery after the battery has reached itsmaximum voltage, then the battery could be overcharged, which could leadto premature battery failure.

Conventional battery chargers have been developed to fully charge aLi-Ion battery. Generally, the battery charger uses a constant current,constant voltage schema to charge the battery. A discharged battery isfirst charged at a constant current level in the range of 0.1C to 1Camperes, where C is the battery capacity in amp-hours, until the batteryreaches the desired voltage of about 4.2 volts. At this point, thebattery charger switches to a constant voltage mode, providing thesufficient power to maintain the battery at this final voltage whileproviding additional charging to the battery.

The charging profile for a typical Li-Ion battery is shown in FIG. 1. Aconstant current is applied for a predetermined period. During thisphase, the charging of the Li-Ion battery is generally constant. For atypical battery, this phase lasts somewhat less than one hour. TheLi-Ion battery eventually exhibits a constant voltage near a preferredvoltage prior to attaining a full charge. A constant voltage is thenapplied to the Li-Ion battery. After approximately an hour of chargingwith a constant voltage, the battery has typically attained its maximumcharge.

If the charging of a Li-Ion battery does not follow the charging profileshown in FIG. 1, then there is a risk that the battery will not be fullycharged or that the charging will damage the battery.

The charging of a Li-Ion battery is further complicated because thebattery is often not fully discharged before charging. If some residualcharge remains on the battery, then optimal charging may require someamount of constant current charging followed by constant voltagecharging, or, alternatively, the optimal charging may require onlyconstant voltage charging. For better performance, the battery chargershould provide a mechanism for compensating for the charge state of thebattery.

Charging Li-Ion batteries is especially problematic where inductivecharging is used. In an inductive battery charger, a primary coillocated in the charger provides power to an inductive secondary locatedin the battery. The voltage across the secondary is then rectified andapplied to the battery to recharge the battery. There is no directphysical connection between the battery and the battery charger. Becausethere is no physical connection between the battery and the batterycharger, information regarding the state of the battery is not readilyavailable to the battery charger.

At the same time, portable devices need to be lightweight. Thus, complexcircuitry to monitor the charge state of the battery and relay thatinformation to the inductive charger increases the cost, size and weightof the portable device.

An inductive system capable of charging a battery having a unique chargecycle while using a relatively simple circuit directly coupled to thebattery is highly desirable.

SUMMARY OF THE INVENTION

An inductive system for recharging a battery, such as a Li-Ion battery,having a unique charging cycle generally includes a charger circuithaving a primary coil for inductively supplying charging power and asecondary circuit for inductively receiving the charging power andapplying that power to a battery. The secondary circuit includes afeedback mechanism to provide feedback to the charger circuit throughthe inductive coupling of the primary coil and the secondary coil. Thecharger circuit includes a frequency control mechanism for controllingthe frequency of the power applied to the primary coil at least partlyin response to the feedback from the feedback mechanism.

In one embodiment, the feedback mechanism includes a subcircuit forvarying the reflected impedance of the secondary circuit. In thisembodiment, the charger circuit may include a feedback detector formonitoring a characteristic of the power in the charger circuit thatvaries at least in part in response to changes in the reflectedimpedance of the secondary circuit. In this embodiment, the feedbackdetector may be coupled to the primary coil to allow the controller tomonitor the current through the primary coil.

In one embodiment, the feedback mechanism includes an over-voltagedetector or an over-current detector, or both. In this embodiment, thedetectors may be arranged so as to control one or more switches, such astransistors. If either an over-voltage condition or an over-currentcondition is detected in the secondary circuit, then the switch isturned on and the current from the secondary coil is shunted from thebattery through a resistor. In this way, the battery is protected fromsignificant exposure to over-voltage or over-current conditions. Thefeedback mechanism may be directly coupled to the battery.

In one embodiment, the feedback detector is a current sensor coupled tothe primary tank circuit. In this embodiment, when the current isshunted through the feedback signaling resistor in the secondary, thecurrent through the secondary coil increases, which varies the reflectedimpedance of the secondary circuit resulting in increased currentthrough the primary coil. The increase in current through the primarycoil is detected by the current sensor in the primary circuit, whichcould include a peak detector, thereby providing to the controller afeedback signal for detecting whether the battery is in an over-voltageor over-current state. In one embodiment, the frequency controlmechanism makes appropriate adjustments to the frequency to correct theover-voltage or over-current state by reducing the power supplied to thesecondary coil.

In one embodiment, the charger circuit includes an inverter and a tankcircuit. In this embodiment, the operating frequency of the inverter isreduced to move the frequency of the power applied to the primary coilcloser to the resonant frequency of the tank circuit, while theoperating frequency of the inverter is increased in order to move thefrequency of the power applied to the primary coil away from theresonant frequency of the tank circuit. It would be equally possible toarrange the system such that an increase in the inverter frequency wouldmove the power applied to the primary coil closer to resonance andthereby increase power transfer while a decrease in inverter frequencywould move the power applied to the primary coil farther from theresonant frequency of the tank circuit and thereby decrease the powertransfer.

In another aspect, the present invention also provides a method foroperating an inductive charging system having a charger circuit with aprimary coil and a secondary circuit with a secondary coil. The methodgenerally includes the steps of detecting whether a battery is presentin the secondary circuit and charging the battery by way of one or morecharging cycles. The charging step generally includes the steps of:applying power to a primary coil at a frequency, evaluating feedbackfrom the secondary circuit through the primary coil and secondary coilinductive coupling and adjusting the frequency of the power applied tothe primary coil as a function of the feedback from the secondarycircuit. In this way, the frequency of the power applied to the primarycoil is changed in order to optimize the charging for the battery.Several charging cycles may be necessary to fully charge the battery.

In one embodiment, the method is utilized with a charger circuit havinga tank circuit. In this embodiment, the charging cycle may include thealternative steps of moving the power applied to the primary coil closerto the resonant frequency of the tank circuit or moving the powerapplied to the primary coil farther from the resonant frequency of thetank circuit. The charger circuit may include an inverter. In suchembodiments, the steps of moving the frequency of the power applied tothe primary coil may be further defined as moving the operatingfrequency of the inverter.

In one embodiment, the charging step generally includes the steps of:applying power to the primary coil at a specific frequency; determiningwhether a feedback signal from the charger circuit is received by thecharger circuit; and varying the frequency of the power applied to theprimary coil as a function of the feedback signal to either increase ordecrease the power delivered to the secondary circuit. In oneembodiment, the step of varying the frequency is further defined asincluding the steps of: moving the frequency of the power applied to theprimary coil farther from resonance if a feedback signal from thefeedback mechanism is received or moving the frequency of the powerapplied to the primary coil closer to resonance if a feedback signalfrom the feedback mechanism is not received.

In one embodiment, the charging step generally includes the steps ofincrementally moving the frequency of the power applied to the primarycoil closer to resonance until a feedback signal is received; after afeedback signal is received, incrementally moving the frequency of thepower applied to primary close farther from resonance until a feedbacksignal is no longer received; and applying power to the primary coil atthe frequency for a charging period. The process may be repeated.

In one embodiment, the step of determining whether a feedback signal isreceived includes the steps of sensing the current in the chargercircuit and comparing the sensed current with a predetermined threshold.

In one embodiment, the method further includes the step of terminatingthe charging cycle when the time for completing one charging cycle isless then a minimum charging cycle time. The method may further includethe step of terminating the charging cycle when the frequency of thepower applied to the primary coil meets an upper and/or lower threshold.

In one embodiment, the detecting step further includes the steps of:applying a pulse of power to the primary coil at a predetermined probefrequency, sensing the reflected impedance, and determining whether abattery is present as a function of the sensed reflected impedance.

The present invention provides a simple and effective inductive chargingcircuit that permits nonlinear charging profiles to be implemented in aninductive system with a small number of components. The over-current andover-voltage detectors not only provide feedback used to drive thefrequency of the charging power, but also protect the battery frompotential harmful power conditions. The charging profile can be easilychanged by varying a number of stored values that dictate operation ofthe system. The present invention is well-suited for use in chargingportable electronic devices, such as cell phones, personal digitalassistants, handheld gaming devices, personal media players and othersimilar devices. In this context, the secondary circuit may beincorporated into the portable electronic device so that the device canbe placed in close proximity to the charger circuit for charging,thereby eliminating the need to plug the device into a charger.

These and other objects, advantages, and features of the invention willbe more fully understood and appreciated by reference to the descriptionof the current embodiment and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the charging profile for a typical Li-Ion battery;

FIG. 2 shows a system for inductive charging of a battery;

FIG. 3 is a power transfer curve between a primary coil and a secondarycoil;

FIG. 4 shows a circuit diagram corresponding to the block diagram ofFIG. 2 for the charger circuit;

FIG. 5 shows a circuit diagram corresponding to the block diagram ofFIG. 2 for the battery side;

FIG. 6 shows the output of the peak detector caused by the increasedcurrent through the secondary coil; and

FIG. 7 is a flow chart for a method of operating a battery charger.

DESCRIPTION OF THE CURRENT EMBODIMENT

An inductive charging system in accordance with an embodiment of thepresent invention in shown in FIG. 2. The inductive charging system 4 isconfigured to inductively charge a battery having a nonlinear chargingprofile, such as a Li-Ion battery. The system 4 generally includes acharger circuit 6 and secondary circuit 8. The charger circuit 6generally includes a primary coil 15, a frequency controller 80 forapplying power to the primary coil at a desired frequency and a feedbackdetector 82 for receiving feedback from the secondary circuit 8. Thesecondary circuit 8 generally includes a secondary coil 30 for receivinginductive power from the charger circuit 6 and a feedback mechanism 84for providing feedback to the charger circuit 6 indicative of thevoltage or current in the secondary circuit 8. The frequency controller80 varies the frequency of the power applied to the primary coil 15 as afunction of the feedback from the secondary circuit 8. Althoughdescribed in connection with the charging of a conventional Li-Ionbattery, the present invention is well suited for use in charging othertypes of batteries, including batteries having different chargingprofiles.

As noted above, the charger circuit 6 generally includes a frequencycontroller 80, a primary coil 15, and a feedback detector 82. In theillustrated embodiment, the frequency controller 80 includes acontroller 20, an oscillator 18, a driver 16 and an inverter 10. In thisembodiment, these components collectively drive a tank circuit 12. Morespecifically, the inverter 10 provides AC (alternating current) power tothe tank circuit 12 from a source of DC (direct current) power 14. Thetank circuit 12 includes the primary coil 15. The tank circuit 12 may beeither a series resonant tank circuit or a parallel resonant tankcircuit. In this embodiment, the driver 16 provides the signalsnecessary to operate the switches within the inverter 10. The driver 16in turn operates at a frequency set by the oscillator 18. The oscillator18 is, in turn, controlled by the controller 20. The controller 20 couldbe a microcontroller, such as a PIC18LF1320, or a more general purposemicroprocessor. Although shown as essentially discrete devices in theillustrated embodiment, the driver 16, oscillator 18 and couldalternatively be integrated and could be implemented as modules withinthe controller 20.

In the illustrated embodiment, the feedback detector 82 detects thecurrent in the tank circuit 12. In operation, the controller 20 usessignals from the peak detector 22 to assist in determining the frequencyof operation for the oscillator 18, and thereby the frequency of theinverter 10. Although the feedback detector 82 of the illustratedembodiment detects current in the tank circuit 12, other characteristicsof the power in the charger circuit 6 may be evaluated to provide chargeinformation regarding the battery.

The secondary circuit 8 generally includes a secondary coil 30, arectifier 32, and a feedback mechanism 84. The secondary coil 30inductively receives power from the primary coil 15. The rectifier 32provides DC power to charge the battery 34. In this embodiment, thefeedback mechanism 84 is configured to provide feedback when the currentapplied to the battery 34 exceeds a threshold value or when the voltageapplied to the battery 34 exceeds a threshold value. As shown, thefeedback mechanism of this embodiment generally includes an over-voltagedetector 36, an over-current detector 40, an OR gate 38, a switch 42,and a resistor 44. The output of an over-voltage detector 36 indicateswhether the voltage across the battery 34 is above a predeterminedlevel. Similarly, the output of an over-current detector 40 indicateswhether the current to the battery 34 is above a predetermined amount.The output of the current detector 40 as well as the output of thevoltage detector 36 are coupled to the inputs of an OR gate 38. The ORgate 38 could be a discrete circuit, or it could be a connection betweenthe outputs of the detectors. The output of the OR gate 38 is coupled tothe switch 42. The switch 42 is controlled by the output of the OR gate38 and is connected in series between the rectifier 32 and the resistor44. The switch 42 could be any suitable switch such as a bipolartransistor, a field effect transistor, or an insulated gate bipolartransistor. The resistor 44 is connected in series between the switch 42and ground.

In operation, if the output of the over-voltage detector 36 or theoutput of the over-current detector 40 indicate an over-voltage or anover-current situation, then the output of the OR gate 38 turns on theswitch 42. When the switch 42 is on, current from the rectifier 32 flowsthrough the resistor 44 to ground.

Because the impedance of the resistor 44 is much less than the impedanceof the battery 34, a current surge occurs through the resistor 44,thereby causing a current surge through the secondary coil 30. The diode64 prevents the battery 34 from supplying any current when the switch 42is turned on. The current surge through the secondary coil 30 creates asimilar current surge in the charger circuit 6 through the primary coil15. The current surge is detected by the peak detector 22. Thecontroller 20 then changes the frequency of the oscillator 18.

In the illustrated embodiment, the primary coil 15 and the secondarycoil 30 are loosely coupled. Because the two are loosely coupled in thisembodiment, the slope of the power transfer curve about the resonantfrequency is not as steep as if the coils 15, 30 were tightly coupled.An exemplary power transfer curve for the coils 15, 30 is shown in FIG.3. In this embodiment, the power transfer is highest when the inverter10 is operating at resonance. However, even if the inverter 10 is notoperating at resonance, significant power transfer can occur when theinverter is operating off-resonance. Generally, the inverter 10 isoperated between frequency A and frequency B. Frequency B is somewhatless than the resonant frequency. Between frequency A and frequency B,the power transfer curve can be piece-wise linearized by a softwarelookup table located in the controller 20. Thus, a decrease in theoperating frequency of the inverter 10 will result in a generallypredictable increase in the power transferred from the primary coil 15to the secondary coil 30. As can be seen by the graph, it would beequally effective to use an operating frequency less than the resonantfrequency. If so, then an increase in the operating frequency would leadto an increase in the power transfer, and vice-versa.

FIG. 4 shows a circuit diagram corresponding to the block diagram ofFIG. 2 for the charger circuit 6 of the system. The peak detector 22(comprised of 22A and 22B) is connected in series with the primary coil15 and provides a signal by way of the transformer 50 that isproportional to the current through the primary coil 15. The signal isrectified by a diode 52 and then used to charge capacitor 54. Theoperational amplifiers 56, 58 are used to smooth the signal for samplingby the controller 20. This particular circuit diagram is exemplary andis not intended to limit the scope of the invention to a specificcircuit design.

FIG. 5 shows a circuit diagram for the secondary circuit 8 of thesystem. As with the charger circuit diagram of FIG. 4, the secondarycircuit diagram of FIG. 5 is exemplary and not intended to limit thescope of the invention to a specific circuit design. Power from thesecondary coil 30 is used to charge capacitor 60, which in turn is usedas the power supply for the circuitry connected to the battery 34. Arectifier 32 produces a DC current from the AC current supplied bysecondary coil 30. A capacitor 62 is charged to provide a DC powersource for charging the battery 34. A blocking diode 64 prevents thebattery 34 from discharging when the secondary coil 30 is not receivingpower or when the feedback mechanism is signaling an over-voltage orover-current condition.

If either the over-voltage detector 36 or the over-current detector 40determines that too much voltage or too much current is applied to thebattery 34, then the transistor 42 is turned on, thereby discharging thecapacitor 62 through the resistor 44, resulting in a lower voltageacross the battery 34. In this embodiment, the secondary circuit 8includes a blocking diode 64 that prevents current from flowing into thecapacitor 62 from the battery 34.

When the current flows through the resistor 44, additional current isdrawn from the secondary coil 30, which in turn causes an increase incurrent through the primary coil 15.

Because the voltage drops across the capacitor 62, the voltage acrossthe battery 34 drops as does the current through battery 34. Thus, theover-voltage condition or the over-current condition is corrected. Thedetectors 36, 40 are cleared, thereby causing the transistor 42 to turnoff. The period the transistor 42 is turned on due to an over-current oran over-voltage condition to the time the transistor is turned off dueto a correction of the over-current or over-voltage condition is thesignal time.

In this illustrated circuit design, the duration of the signal time iscontrolled by two RC circuits 66, 68 within the detector circuits 36,40. In this embodiment, the voltage detector 36 is configured to have ahysteresis of about 80 mV to reduce oscillation when the battery 34voltage is near an over-voltage condition.

As stated, when the transistor 42 is turned on, increased current flowsthrough the secondary coil 30, causing increased current flow throughthe primary coil 15. This increase in current is detected by the peakdetector 22. The output of the peak detector caused by the increasedcurrent through the primary coil is shown in FIG. 6. The output of thepeak detector in the illustrated embodiment increases by about 1.55V forabout 10 ms. The characteristics of this signal may vary fromapplication to application depending on the characteristics of thecircuit components. For example, the magnitude of the increase and thelength of the increased signal may be controlled as desired.

The controller 20 continuously samples the output of the peak detector22. When a sudden increase is detected, an internal flag referred to asFB_flag is set. When a decrease is detected, FB_flag is cleared.However, a copy of FB_flag referred to hereinafter as FB_latch is alsoset. FB_latch is not cleared when a decrease is detected. It can only becleared by the controller 20. FB_latch therefore can be checkedperiodically by controller 20 to determine whether an over-voltagecondition or an over-current condition occurred during a given period oftime. Thus, the system provides a feedback mechanism to the controller.

A user of a portable device may remove the device from the chargercircuit 6 before it is fully charged. Additionally, the user may placethe device in the charger before the battery is full discharged. Inorder to optimally charge the battery, the inductive battery charger maydetect the presence of the battery as well as compensate for the uniquecharging profile for a battery.

FIG. 7 is a flow chart showing one embodiment of the process fordetecting whether the secondary circuit 8 is proximal to the chargercircuit 6 and for optimally charging the battery if the secondarycircuit 8 is present.

The process starts. Step 100. The probing process 99 is commenced. Thecontroller 20 waits for a predetermined time period of PROBE_INTERVAL.Step 102. After PROBE_INTERVAL has elapsed the controller 20 causes theinverter 10 to produce a low frequency current at PROBE_FREQUENCYthrough the primary coil 15. Step 104. The current through the primarycoil 15 is detected. Step 106.

If the secondary circuit 8 is present, then the probe by the chargercircuit 6 will induce a probe current in the secondary coil 30. Thebattery 34 will not be damaged even if it is fully charged at the timeof the probe. First, the probe is of a short duration on the order of 10to 20 milliseconds, in this embodiment, while the quiescent period isusually several seconds long. Additionally, the over-voltage detector 36and the over-current detector 40 by way of transistor 42 will shuntexcessive probe current through the resistor 44 rather than through thebattery 34.

In this embodiment, the amount of current through the primary coil atthe PROBE_FREQUENCY has been previously determined experimentally andsaved into the memory of the controller. If the current through theprimary coil 15 is approximately equal to the predetermined unloadedprimary current (Step 108), then the secondary circuit 8 is not presentin the secondary circuit 8. The CHARGED_FLAG is cleared. Step 109. Thesystem then waits for another PROBE_INTERVAL before starting the processagain.

If the current flowing through the primary coil 15 is not approximatelyequal to the predetermined unloaded primary current, then the secondarycircuit 8 is present.

The CHARGED_FLAG is then checked. Step 111. The CHARGED_FLAG indicateswhether the battery is fully charged. If the CHARGED_FLAG is not set,then the charging process begins.

The frequency of the inverter is set by the controller 20 to FREQ_START.Step 110. The system then delays for a predetermined period of time toeliminate any transients. Step 112.

The controller 20 then determines whether a feedback signal, discussedpreviously, has been received. If not, then the frequency is decreasedby ΔFREQ. Step 116. In this embodiment, a decrease in the frequencymoves the system toward resonance, and therefore increases the powertransfer from the charger circuit 6 to the battery 34.

ΔFREQ could be a constant, or it could be determined by obtaining thevalue from a lookup table indexed by the operating frequency of theinverter at the particular time ΔFREQ is used. The values selected forΔFREQ may be frequency dependent and chosen so that if the operatingfrequency is reduced or increased by ΔFREQ, then the correspondingincrease or decrease in current is the approximately the same for ΔFREQsfor all operating frequencies. For example, if the charger circuit 6 isoperating near the resonant frequency of the tank circuit 12, then adecrease in the operating frequency by 100 Hz will substantiallyincrease the current through the tank circuit 12. If, on the other hand,the charger circuit 6 is operating relatively far from resonance, then achange of 100 Hz will not result in a substantial increase in thecurrent through the primary. ΔFREQ may therefore be chosen to causeapproximately the same change in primary current at a low frequency or ahigh frequency

The frequency is then compared with Min_FREQ. Step 118. Min_FREQ is thepredetermined minimum operating frequency for the inverter. Generally,Min_FREQ is somewhat greater than the resonant frequency for the tankcircuit 12. If the frequency is less than or equal to that of Min_FREQ,then the controller 20 returns to probing. If not, then the controller20 waits for a predetermined period time (Step 112) and then checks forthe occurrence of a feedback signal. Step 114.

Thus, as long as no feedback signal is detected by the controller 20,the frequency of the inverter 10 is repeatedly reduced so as to maximizepower transfer to the battery 34.

If a feedback signal is detected, then the power transfer to the battery34 should be reduced. The frequency is therefore increased by an amountequal to twice the value of ΔFREQ, which again could be obtained from alook-up table. Step 122. The frequency is compared with Max_FREQ. Step124. Max_FREQ is a predetermined value indicating the maximum frequencyfor operating the inverter. If the frequency will be greater than apredetermined maximum frequency Max_FREQ, then the charger circuit 6returns to the probing process 99. If not, the controller 20 waits (step126) and then checks for a feedback signal. Step 128.

If a feedback signal has been detected, then the inverter frequency isagain decreased by twice the value of ΔFREQ. Step 122. The process thencontinues. On the other hand, if no feedback signal is detected, thenthe system waits while power at the then-current frequency is applied tothe primary coil 15. Step 130. The long charge delay of step 130 isgenerally much larger than the delays of step 112 or step 126. The longcharge delay allows a substantial amount power to be provided to thebattery 34.

Thus, as the charge on the battery 34 is increased and a feedback signalis detected, then the system gradually increases the operating frequencyof the inverter 10, thereby reducing the power transferred to thebattery 34. The increase in the operating frequency continues untilfeedback signals are no longer received, in which case power is providedto the battery 34 over a longer period of time, thereby allowing thebattery 34 to charge to a maximum.

Returning to step 124, if the operating frequency is greater than theMax_FREQ, then the controller 20 compares CHARGE_TIME withMIN_CHARGE_TIME. Step 132. CHARGE_TIME is the length of elapsed time forthe previous charging cycle, while MIN_CHARGE_TIME is the minimumdesired time for a charge cycle. If the CHARGE_TIME is less than theMIN_CHARGE_TIME, then the battery 34 is considered to be fully charged,and the CHARGED_FLAG is then set. Step 134. Additionally, an LED may beturned on to indicate to a user that the battery 34 is fully charged.

The system may be configured to address fault conditions. In oneembodiment, the controller 20 may include a counter that is incrementedeach time an entire charge cycle occurs without generating a feedbacksignal. When the value of the counter is greater than the predeterminedmaximum number of faults, the system enters an irrevocable fault state.The controller 20 may then deactivate the drive signal, and may enable ared LED to flash rapidly. In this embodiment, the charger circuit 6 canonly be returned to operation by power cycling the charger. That is, thecharger circuit 6 must be disconnected from the external power source.

Further, if the feedback drives the frequency above a predetermined safefrequency, designated as FREQ_TRIGGER_SAFE, then the minimum frequencyis set to FREQ_MIN_SAFE. If the algorithm would take it lower than thislevel, the system continues to probe as usual. If there is a faultwithin the system, the fault condition will occur and the chargercircuit 6 will be disabled until the charger circuit 6 is power cycled.

Although the present invention is described in connection with anembodiment in which changes in the impedance of the secondary circuit(for example, resulting from changes in resistance) are used to generatea feedback signal, the present invention is not limited to the feedbackmethodology of the illustrated embodiment. The present invention mayutilize, among other things, changes in resistance, capacitance and/orinductance in series or parallel configurations to generate the feedbacksignal.

The above descriptions are those of current embodiments of theinvention. Various alterations and changes can be made without departingfrom the spirit and broader aspects of the invention.

1. A method of operating an inductive charger circuit for supplyingpower to an object via an inductive coupling, the inductive chargercircuit having a primary circuit, said method comprising the steps of:probing for presence of the object by supplying power to the primarycircuit and sensing an amount of power supplied to the primary circuit;determining if the sensed amount of power is approximately equal to apredetermined threshold; based on the sensed amount of power beingapproximately equal to the predetermined threshold, setting a chargestatus to indicate the battery is not fully charged; checking the chargestatus; supplying power to the primary circuit at an operating frequencyin order to charge the battery of the object if (i) the sensed amount ofpower is not approximately equal to the predetermined threshold and (ii)the charge status indicates the battery is not fully charged; andrepeating said probing and said determining steps if (i) the sensedamount of power is not approximately equal to the predeterminedthreshold and (ii) the charge status indicates the battery is fullycharged.
 2. The method of claim 1, wherein said probing includessupplying power to the primary circuit at a predetermined probingfrequency.
 3. The method of claim 1, wherein the charge status is astatus of a charge flag.
 4. The method of claim 1, wherein thepredetermined threshold is an unloaded primary current.
 5. The method ofclaim 1, wherein the sensed amount of power is approximately greaterthan the predetermined threshold when the object is in proximity to theinductive charger circuit.
 6. The method of claim 1, wherein the objectis a secondary circuit having the battery for charging via the inductivecoupling.
 7. The method of claim 1 further including the step ofreceiving a feedback signal from the secondary circuit in response to asensed over-voltage condition of the battery or a sensed over-currentcondition of the battery.
 8. The method of claim 7, wherein the objectincludes a switch to generate the feedback signal to the inductivecharging circuit via the inductive coupling, wherein activation of theswitch varies a reflected impedance.
 9. An inductive charger circuit forsupplying power to an object via an inductive coupling, said inductivecharger circuit comprising: a power source; a primary circuit includinga primary for supplying wireless power to said object via an inductivecoupling; a sensor for providing an output indicative of an amount ofpower in said primary; and a controller electrically coupled to saidprimary circuit and electrically coupled to said power source, saidcontroller configured to apply power at a frequency to said primary fromsaid power source; said controller configured to probe for presence ofsaid object by applying power to said primary; said controllerconfigured to determine, based on said sensor output, if said amount ofpower is approximately equal to a predetermined threshold in response toapplication of power, said controller configured to set a charge statusto indicate said battery is not fully charged if said sensed amount ofpower is approximately equal to said predetermined threshold, whereinpower is supplied to said primary circuit at an operating frequency inorder to charge said battery of said object if (i) said sensed amount ofpower is not approximately equal to said predetermined threshold and(ii) said charge status indicates said battery is not fully charged,wherein if (i) said sensed amount of power is not approximately equal tosaid predetermined threshold and (ii) said charge status indicates saidbattery is fully charged, said controller repeats application of powerto said primary and determines, based on said sensor output, if saidamount of power is approximately equal to said predetermined threshold.10. The inductive charger circuit of claim 9, wherein said controllerprobes for presence of said object by applying power to said primary ata probing frequency.
 11. The inductive charger circuit of claim 9,wherein said predetermined threshold is an unloaded primary current. 12.The inductive charger circuit of claim 9, wherein said sensed amount ofpower is approximately greater than said predetermined threshold whensaid object is in proximity to said inductive charger circuit.
 13. Theinductive charger circuit of claim 9, wherein said sensor is a currentsensor, and said sensor output is indicative of a current in saidprimary.
 14. The inductive charger circuit of claim 9, wherein saidcharge status is a status of a charge flag.
 15. The inductive chargercircuit of claim 9, wherein said object is a secondary circuit having abattery for charging via said inductive coupling.
 16. The inductivecharger circuit of claim 15, wherein said secondary circuit generates afeedback signal in response to a sensed over-voltage condition of saidbattery or a sensed over-current condition of said battery.
 17. Theinductive charger circuit of claim 15, wherein said object includes aswitch to generate a feedback signal to said inductive charging circuitvia said inductive coupling, wherein closing of said switch varies areflected impedance.
 18. The inductive charger circuit of claim 17,wherein in response to said switch being closed, a current is shuntedthrough a resistive element of said object, said current results in achange in reflected impedance of said object and an increase in currentthrough said primary, and wherein said sensor detects said increase incurrent through said primary.